Database diagram support objects cannot be installed because this database does not have a valid owner

I got the following error message when I clicked on database diagrams in Microsoft SQL Server Management Studio 2005.

Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.

I have upgrade the database to the level 90 by the following step but it is not working.

In SQL Server Management Studio do the following:

1. Right Click on your database, choose properties
2. Goto the Options Page
3. In the Dropdown at right labeled "Compatibility Level" choose "SQL Server 2005(90)"

I got it working after I execute the following command to transfer the database ownership to sa

ALTER AUTHORIZATION ON DATABASE::DatabaseName TO sa